Here are some interesting thoughts on the topic by @remlaps:
https://steemit.com/hive-101145/@remlaps/re-steemchiller-q91w4e

As I wrote recently:

Of course, there are various reasons for downvoting content and maybe we won't need it for normal content in future. The way the old witnesses have gone (for example, automatically downvoting people based on the used number of words in a post) clearly was not the best one.
 
I think we should mainly focus on real abuse, cases where someone posts 10 senseless comments a day and upvotes each of them 100% with a $5 vote. Of course, the same goes for trashy root posts. But I think we will never find a working measurement instrument for quality content, because people are just too different.